INTRACELLULAR ACIDIFICATION IS ASSOCIATED WITH, BUT NOT REQUIRED FOR CASPASE ACTIVATION, DNA FRAGMENTATION OR APOPTOSIS

Apoptosis is characterized by DNA digestion mediated by either a Ca2+/ Mg2+-dependent endonuclease or the acid-activated deoxyribonuclease II (DNase II). However, DNA digestion frequently does not correlate with changes in Ca2+ whereas intracellular acidification is a consistent m arker of apoptosis. To confirm the role of low pH in regulating DNA di gestion, ML-I cells were damaged with etoposide then incubated at vari ous extracellular pH (pH,). When pH, was 8.1, DNA digestion still occu rred, and intracellular pH still decreased but only to 7.2, a pH at wh ich DNase LT is inactive. In contrast, low pH, inhibited the DNA diges tion and apoptosis induced by etoposide. An upstream event in apoptosi s is the activation of proteases known as caspases. The activity of ca spases was inhibited at low pH, demonstrating that the pH-sensitive st ep is upstream of caspase action. Similar results have been obtained i n other models of apoptosis. Hence, both DNase II and Ca2+/Mg2+-depend ent endonuclease appear unlikely to cause DNA digestion in apoptosis, unless their ion dependence is modified by, for example, proteolytic c leavage.